I will assume you have basic knowledge of Creation Kit, how to set up you save files and that, but if you don't just post and I will add that.

You need to open up the item of clothing in Creation Kit. I would recommend making a duplicate/new form so that that the default remains unaltered.

A box opens up with all the fields that can be edited.

- you need to add the armour rating you want - and set the clothing as light or heavy armour! EDIT: the drop down menu for this is next to weight. - in key words you may want to change a few things. I usually use the same keywords as say leather armour so that the clothing is treated as leather armour by merchants, perk trees etc.

After you have made your new armour you will need to add it to the game with a recipe or adding it to a shopkeepers inventory or placing it somewhere. I recommend making a crafting recipe. It won't conflict with anything else that way.

You will probably want to add a tempering recipe too.

Recipes are under constructible item easiest way is to duplicate an existing recipe for a simliar piece of armour and change the created item to your item.